1. Field of the Invention
The present invention relates generally to utility pliers and, more particularly, to pliers that self-adjust for applying an initial grasping force to a workpiece and for augmenting the initial grasping force applied to the workpiece.
2. Description of the Prior Art
Many types of handheld utility pliers are known in the art. Conventional pliers typically include two plier members interconnected in a scissor-like arrangement allowing for a workpiece to be grasped by jaw portions of the pliers in response to movement of handle portions of the pliers. Over the years, numerous improvements have been made to the conventional plier design in order to obtain better and more efficient pliers. For example, self-adjusting pliers have been developed in order to provide a set of pliers that more easily and automatically adjust to the size of a given workpiece. However, while such pliers provide adjustment capability, they do not provide active augmentation of clamping force beyond what conventional pliers provide.
A limitation of conventional plier designs is that there is an absolute limit to how close the pivot point can be moved toward the jaws, which also limits the amount of mechanical advantage a user has for applying force to a workpiece. Furthermore, in conventional pliers the handles and jaws are coupled in a fixed relationship, typically using the scissor-like arrangement as described, such that the jaws converge on a workpiece at essentially the same rate as the handles when a user applies hand pressure to the pliers. This type of fixed relationship between the handles, the jaws and the pivot point limits the amount of force that a user can apply to a workpiece and produces an undesirable trade-off between overall handle separation and gripping force being applied to a workpiece. Specifically, the longer the handles the greater the leverage and hence the greater the gripping force that can be applied to the workpiece. However, longer handles are impractical and make use of the pliers more inconvenient because either the handles are too far apart to be conveniently grasped by one hand, or the small jaw opening limits the range of adjustability of the jaws.
U.S. Pat. No. 5,832,793 discloses an adjustable wrench having a movable handle and a movable jaw for adjusting the wrench to grip objects of various sizes. While this wrench provides some degree of increased mechanical advantage as well as adjustability for grasping variously-sized workpieces, size adjustment is not automatic and requires discrete manipulations using two hands.
U.S. Pat. No. 2,144,180 discloses adjustable pliers where the handles and jaws are arranged other than in the typical scissor-like arrangement. While these pliers do allow for a level of size adjustment, the function is not provided in a seamless, one-handed operation. These particular pliers require a user to re-position his hand for each step of operation.
Many other types of pliers having handles and jaws coupled in a fixed relationship that limits the amount of force that a user can apply to an object are known. For example, U.S. Pat. No. 4,651,598 discloses utility pliers that provide for self-adjustment through employment of a spring-biased control arm positioned between the handles. In this particular hand tool the range of size adjustability within the envelope of acceptable handle spacing is limited because a large portion of the available handle movement is taken up with moving the jaws up against the workpiece from the fully open rest position. This leaves only a minor portion of available handle movement for carrying out the crucial task of workpiece compression. In addition, U.S. Pat. Nos. 3,232,152, 2,906,155 and 1,651,216 disclose adjustable pliers which utilize the concept of shifting pivot points between first and second pivot means positioned at different locations on the pliers.
U.S. Pat. No. 5,609,080 discloses another type of pliers which is similar to the well known VISE-GRIP type pliers. Such pliers are typically not considered self-adjusting because they must be initially adjusted to set the opening of the jaws in relation to the workpiece to be grasped.
There remains a need for improved handheld utility pliers which provide the capability of applying a greater force to an object being gripped by the pliers and which can be easily operated by the user, preferably with one hand.